The present invention pertains generally to solid propellants and particularly to ballistic modifiers for double-base propellants.
A double-base propellant has an energetic polymer, generally nitrocellulose, plasticized into a gel by an energetic plasticizer, generally nitroglycerine. Various additives are also included in the propellant to improve the mechanical or ballistic properties of the propellant. One such additive is termed a ballistic modifier which alter the inherently high dependence of the burning rate on chamber temperature and especially chamber pressure.
The objective in ballistic modification of doublebase rocket propellants is to obtain plateau or mesa burning over a desired range of pressure and burning rate level. These terms come from the shape of a log-log plot of the burning rate equation for double-base propellants which is given as: r=CP.sup.n or log r=n log P+log C, wherein r is the burning rate, P is the combustion chamber pressure, C is a constant for each propellant composition at any one temperature, and n is a constant for nonmodified propellants but is a variable in modified propellants. In platueau-or mesa-burning propellant, "n" varies from very high positive values to zero or low negative values. Thus, a plot of log r against log P would give a straight line with a slope of "n" for a non-modified propellant, but a "plateau" shaped line or a mesa-shaped line for modified propellants.
The performance of a ballistic modifier is measured in terms of the rate increase and pressure extent of plateau burning. In addition to performance, thermal and hydrolytic stabilities are important criteria for ballistic modifiers.
Of the many ballistic modifiers which have been tested or used to date, none has been proven completely satisfactory. The most widely used ballistic modifiers are lead and cupric salicylates and beta-resorcylates which are disclosed and claimed in U.S. Pat. No. 3,138,499 by A. T. Camp et al. A major difficulty with these modifiers is their hydrolytic instability which necessitates their addition to the propellant composition at a late stage of propellant compounding.
Attempts to develop ballistic modifiers with better performance than the above resorcylates have not been successful. Lead alizarinate and lead quinizarinate were tested extensively and the results have been reported in NAVORD Report 5870 by A. T. Camp et al. Lead alizarinate had an effectivity almost as high as the previous resorcylates, but was more hydrolytically unstable and was toxic. Lead quinizarinate proved to be ballistically ineffective.